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Date:   Thu, 21 Apr 2022 10:06:39 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     Wang ShaoBo <bobo.shaobowang@huawei.com>, cj.chengjian@huawei.com,
        huawei.libin@huawei.com, xiexiuqi@huawei.com, liwei391@huawei.com,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        catalin.marinas@arm.com, will@kernel.org, zengshun.wu@outlook.com
Subject: Re: [RFC PATCH -next v2 3/4] arm64/ftrace: support dynamically
 allocated trampolines
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On Thu, 21 Apr 2022 14:10:04 +0100
Mark Rutland <mark.rutland@arm.com> wrote:

> On Wed, Mar 16, 2022 at 06:01:31PM +0800, Wang ShaoBo wrote:
> > From: Cheng Jian <cj.chengjian@huawei.com>
> > 
> > When tracing multiple functions customly, a list function is called
> > in ftrace_(regs)_caller, which makes all the other traced functions
> > recheck the hash of the ftrace_ops when tracing happend, apparently
> > it is inefficient.  
> 
> ... and when does that actually matter? Who does this and why?

I don't think it was explained properly. What dynamically allocated
trampolines give you is this.

Let's say you have 10 ftrace_ops registered (with bpf and kprobes this can
be quite common). But each of these ftrace_ops traces a function (or
functions) that are not being traced by the other ftrace_ops. That is, each
ftrace_ops has its own unique function(s) that they are tracing. One could
be tracing schedule, the other could be tracing ksoftirqd_should_run
(whatever).

Without this change, because the arch does not support dynamically
allocated trampolines, it means that all these ftrace_ops will be
registered to the same trampoline. That means, for every function that is
traced, it will loop through all 10 of theses ftrace_ops and check their
hashes to see if their callback should be called or not.

With dynamically allocated trampolines, each ftrace_ops will have their own
trampoline, and that trampoline will be called directly if the function
is only being traced by the one ftrace_ops. This is much more efficient.

If a function is traced by more than one ftrace_ops, then it falls back to
the loop.

-- Steve